The present invention generally relates to semiconductor device fabrication. More particularly, the present invention relates to the curing of an adhesive material used in affixing solder masks to semiconductor chips.
Some conventional semiconductor devices include chips having a solder mask and printed or screened-on conductive traces for wirebonding to a ball grid array (BGA). Generally, the solder mask is affixed to the chip by an adhesive material. Typically, the adhesive material is applied to the chip and allowed to cure prior to deposition of the solder mask. Currently utilized adhesive materials cure at a temperature in excess of 150xc2x0 C.
Most solder masks are formed from a liquid photoimageable material. Two popular solvents used in forming liquid photoimageable solder masks are diethylene glycol monoethyl ether acetate (DGMEA) and dipropylene glycol monoethyl ether (DGME). Often a heavy aromatic naphtha also is used as a photoinitiator. All of these materials boil at relatively high temperatures. Specifically, DGMEA boils at 219xc2x0 C., DGME boils at 90xc2x0 C., and naphtha boils at between 80xc2x0 and 220xc2x0 C.
Some currently used fabrication methods cure the adhesive material along with the solder mask. During such methods, a cure of about one hour at 150xc2x0 C. of the liquid photoimageable solder mask is carried out. Such a cure serves to drive the low temperature volatile components of the solder mask, i.e., from the DGMEA and/or DGME, out, leaving behind the higher temperature volatiles to outgas later when the temperature of the device in operation reaches a sufficient outgassing temperature. Since the cure time and temperature are insufficient to cure the adhesive material, later outgassing may induce voids in the adhesive material. Voids are capable of entrapping moisture, causing the semiconductor package to fail an environmental test. Further, outgassing contaminates the bond pads, resulting in a low bond yield. In addition, curing at high temperatures creates thermal stresses between the adhesive material and the die which are particularly problematic for large and/or thin semiconductor device packages.
There exists a need for a curing methodology which inhibits the effects of outgassing on adhesive material, thereby reducing voiding and the collection of moisture within the adhesive material, as well as which reduces thermal stress on the device package and contamination of the bond pads.
The present invention provides a semiconductor device having a solder mask, a die and an adhesive layer affixing the die to the solder mask. The adhesive layer is cured at a temperature below about 100xc2x0 C.
The present invention also provides a semiconductor device having a solder mask, a die, electrical contacts on the solder mask and the die, each contact on the die being wire bonded to a respective contact on the mask, and an adhesive layer affixing the die to the solder mask. The adhesive layer is cured at a temperature between about 20xc2x0 C. and about 50xc2x0 higher than a glassy temperature of the adhesive layer and the curing temperature is below about 100xc2x0 C.
The present invention further provides a semiconductor package including a chip, a solder mask affixed to a die by an adhesive layer which is cured at a temperature below about 100xc2x0 C., the die being electrically connected to the chip, and a mold encapsulating the chip, solder mask and die.
The present invention further provides a method of forming a semiconductor device. The method includes the steps of affixing a solder mask to a semiconductor die with an adhesive layer, and curing the adhesive layer by exposing the adhesive layer to a temperature no greater than 100xc2x0 C.
These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.